1. Field of the Invention
The present invention relates to fiber-reinforced metallic matrix composites, novel coated reinforcing fibers for use therein and methods for producing such fibers and composites.
Fiber-reinforced metallic matrix composites commonly are based upon metal alloy systems having good resistance to oxidation and erosion and good strength properties at elevated temperatures, such as titanium alloys, superalloys, intermetallics, etc., and are used in gas turbine engine compressor and turbine components.
2. Discussion of Prior Art
Reference is made to commonly-assigned U.S. Pat. Nos. 4,816,347, 4,896,815 and 5,024,889 for their disclosure of fiber-reinforced titanium alloy matrix composite materials and methods for producing such composite materials, such as by hot isostatic pressing laminates of interposed titanium alloy layers and layers or fabrics of conventional reinforcing fibers. Such fibers frequently comprise core materials, such as boron or carbon (graphite), which are barrier-coated with compatible ceramic materials, such as boron carbide or silicon carbide, in an effort to insulate them against reaction with the metal matrix layers, such as titanium-aluminum alloys.
Reference is also made to U.S. Pat. Nos. 4,010,884, 4,141,802 and 4,499,156, each of which discloses fiber-reinforced metal matrix composite (MMC) materials incorporating fibers which are coated. The latter patent discloses titanium alloy composites incorporating barrier-coated fibers such as silicon carbide-coated boron and formed under temperature and pressure conditions which reduce the amount of reaction at the interfacial zone between the fiber and the alloy matrix. Such a reaction between the titanium alloy and the fiber material forms TiB, TiC, cracking, etc., resulting in severe degradation of tensile strength, particularly at elevated temperatures.
Problems are encountered with known fiber-reinforced metal matrix composites, particularly cracking and severe degradation of tensile strength at elevated temperatures. Stresses are encountered due to thermal coefficient mismatches and/or chemical reactivity between the fibers, generally of elements including boron, carbon, silicon, beryllium or refractory materials, such as silicon carbide (SIC), aluminum oxide or single crystal sapphire (Al.sub.2 O.sub.3), and the metal-matrix, such as Ti--6Al--4V or alpha-2 (titanium aluminide, Ti--23Al--10Nb--3V--1Mo).
The application of the refractory surface coating to the fiber, to reduce interfacial problems, such as incompatibility and chemical reaction, between the fiber core and the metal matrix produces satisfactory results except under severe conditions of temperature and stress. Known refractory fiber coating materials include compounds such as oxides, nitrides, borides, silicides and carbides of elements such as silicon, boron, titanium, aluminum, etc. However, such barrier layers generally are unsatisfactory at elevated temperatures, due to their unsatisfactory bonding properties to the metal matrix, and/or thermal expansion mismatch resulting in delamination or disbonding, cracking and severe degradation of the tensile strength of the composite during thermal cycling.
Reference is made to commonly-assigned U.S. Pat. Nos. 5,024,889; 4,628,002; 4,415,609; 4,340,636; 4,315,968 and 4,142,008 each of which discloses the preparation of reinforcing ceramic fibers such as silicon carbide by the vapor deposition of coatings to a fiber core for purposes of improving the strength, bonding properties, inertness and/or other properties of the fiber when incorporated into a metla matrix for reinforcement purposes.
Advanced gas turbine engines require new materials that can be used under severe temperatures and mechanical stresses. These demanding requirements limit the choice of materials to very few candidates. Fiber-reinforced composites based on titanium alloys and intermetallics are candidates for such applications because of their low densities and high temperature capabilities. However, several fundamental problems must be solved for the successful application of these materials.
The fiber and matrix must retain useful mechanical properties at high temperatures, and must possess chemical compatibility inside the composite. Fiber reaction with the matrix at high temperatures often leads to the formation of an interfacial reaction zone, which causes deterioration in the mechanical strength of the composite. Thermal expansion mismatch between the fiber and matrix can result in matrix cracking in the interface region that results in loss of performance.
All of the critical problems involve processes occurring in the interfacial region between the fiber and matrix. Since the choice of fibers is very limited, the development of new fiber coatings provides a means to control the properties of the interfacial region.
The objective of the present invention is to provide novel intermediate layers (between fiber and matrix) in minimizing stress due to thermal expansion mismatch between the fiber and matrix.
Thus there is a need for improved coated ceramic reinforcing fibers for titanium alloy and intermetallic matrix composites, such as improved silicon carbide fibers, having strong bonding affinity for titanium alloy and intermetallic metal matrix materials over a wide range of temperatures, and which provide a stable barrier interface between the fiber and the metal matrix, preventing chemical reaction and other interfacial problems therebetween, particularly at elevated temperatures.